Auto merge of #106340 - saethlin:propagate-operands, r=oli-obk

Always permit ConstProp to exploit arithmetic identities

Fixes rust-lang/rust#72751

Initially, I thought I would need to enable operand propagation then do something else, but actually rust-lang/rust#74491 already has the fix for the issue in question! It looks like this optimization was put under MIR opt level 3 due to possible soundness/stability implications, then demoted further to MIR opt level 4 when MIR opt level 2 became associated with `--release`.

Perhaps in the past we were doing CTFE on optimized MIR? We aren't anymore, so this optimization has no stability implications.

r? `@oli-obk`

Author: bors